Recently applications of lasers have been expanding to ultraviolet ray lithography, laser micromachining, laser fusion, etc., and laser oscillation apparatuses for use therein need to efficiently produce stable ultraviolet light. All-solid-state laser oscillation apparatuses using nonlinear optical crystals for converting the wavelength of light sources to obtain ultraviolet light have attracted much attention as one way of meeting this need. The nonlinear optical crystals need to have higher output and improved laser damage resistance in order to more efficiently obtaining ultraviolet light.
As nonlinear optical crystals for wavelength conversion to ultraviolet light, for example, lithium-borate-based crystals (LBO crystals: LiB3O5) have been adapted for practical use in converting Nd:YAG laser or Nd:YVO4 laser, wavelength 1064 nm, light to the third harmonic, wavelength 355 nm. However, the LBO crystals have poor surface deterioration and poor wavelength conversion efficiency, so that there has been strong demand for a novel alternative nonlinear optical crystal that does not easily deteriorate and has a high wavelength conversion efficiency. The inventors and other researchers have made extensive studies, and as a result, we have come to see hope in cesium-borate-based crystals (CBO crystals: CsB3O5 as a nonlinear optical crystal alternative to the LBO crystals. The CBO crystals have 2 or more times as large nonlinear optical constant as the LBO crystals and can efficiently generate the third harmonic of the Nd:YAG laser or the Nd:YVO4 laser, whereby they can be put into practical use as high-performance wavelength conversion nonlinear optical crystals. Further, cesium-lithium-borate-based crystals (CLBO crystals: CsLiB8O10) proposed by the inventors can produce a shorter wavelength light such as fourth harmonic, wavelength 266 nm, of Nd:YAG laser light, converting this light with a high conversion efficiency and wide temperature and angle tolerance ranges, and we have great expectations for these novel high-performance wavelength conversion nonlinear optical crystals (Reference 1).
Conventionally, borate-based crystals such as CBO crystals and the CLBO crystals are obtained as single crystals by heating and melting carbonate powder, the raw material, and by growing crystals from the obtained crystal growth material using various crystal growth methods. For example, in the case of the CBO crystals, the inventors have directly mixed and heat-melted 26.6 mol % of Cs2CO3 and 73.3 mol % of B2O3, and then grown the CsB3O5 crystal using a Kyropoulos technique. However, this method is disadvantageous in that it is not easy to homogenize the composition of the melt, added inclusions are incorporated throughout the crystal volume during the crystal growth to cause strong light scattering and poor laser-induced damage tolerance (Reference 2).
Thus, the inventors then mixed 30 mol % of Cs2CO3 and 70 mol % of B2O3, heat-melted the mixture, and grew CsB3O5 using the combination of a TSSG technique and a solution stirring technique comprising putting a propeller into a solution and rotating the crucible. Though the method can remarkably reduce the light scattering in the grown crystal, the liquid is significantly evaporated near the liquid surface, so that it is difficult to achieve the crystal growth itself, and the crystal cannot be grown over a long period (Reference 3).
In any of the above methods, the starting material powders are directly mixed and used as the growth material for crystal growth, so that the powders generate bubbles due to decarboxylation in the heat melting and swell in the reaction. To prevent this, the powders are mixed and melted gradually in several separate operations. Thus, in the production of the CBO crystals and the CLBO crystals, the starting material powders are beat-melted over several days in a melting furnace and then cooled to ordinary temperature, the resultant crystal growth material is transferred to a growing furnace and melted, and single crystals are grown from the melt. The conventional production of the CBO crystals and the CLBO crystals is thus disadvantageously time-consuming and costly.
The present invention was designed after considering the above, and an object of the invention is to overcome these conventional problems, thereby providing a novel method of easily producing a high-quality borate-based crystal excellent in uniformity, which is useful for optical wavelength conversion device, etc, at low cost in a short period of time, and a laser oscillation apparatus using the crystal as an optical wavelength conversion device    Reference 1: Y. Mori, et al., “New nonlinear optical crystal: Cesium lithium borate”, Appl. Phys. Lett, 67, 13 (1995) 1818,    Reference 2: Y. Kagebeyashi, Y. Mori and T. Sasaki, “Crystal growth of cesium triborate, CsB3O5 by Kyropulos technique”, Bulletin of Materials Science, Vol. 22 (6) pp. 971-973, 1999    Reference 3: R. Kitano “Efficient 355-nm generation in CsB3O5 crystal”, Optics Letters, Vol. 28, No. 4, pp. 263-265, 2003.